Logging tools may be used in a wellbore to make, for example, formation evaluation measurements to infer properties of the formation surrounding the wellbore and the fluids contained therein. Some logging tools, known as wireline (WL) tools, are conveyed into a wellbore on a wireline cable, after the wellbore had been drilled. Other logging tools, known as logging-while-drilling (LWD) tools, can be included within a drill string, and provide for the collection of real-time or near real-time information while drilling a wellbore.
One particularly useful type of logging tool that may be embodied as either a WL tool or a LWD tool utilizes Nuclear Magnetic Resonance (NMR) to measure the response of nuclear spins in formation fluids to applied magnetic fields, and from that response evaluates various properties of the formation. These NMR tools are desirable for the accuracy of the measurements that can be made therewith, as well as for the more permissive regulatory environment surrounding NMR than some other technologies.
Such NMR tools typically have a permanent magnet that produces a static magnetic field at a desired test location (e.g., where the fluid is located). The static magnetic field produces an equilibrium magnetization in the fluid that is aligned with a magnetization vector along the direction of the static magnetic field. A transmitter antenna produces a time-dependent radio frequency magnetic field that is perpendicular to the direction of the static field. The radio frequency magnetic field produces a torque on the magnetization vector that causes it to rotate about the axis of the applied radio frequency magnetic field. The rotation results in the magnetization vector developing a component perpendicular to the direction of the static magnetic field. This causes the magnetization vector to align with the component perpendicular to the direction of the static magnetic field, and to precess around the static field.
The time for the magnetization vector to re-align with the static magnetic field is known as the longitudinal magnetization recovery time, or “T1 relaxation time”.
The spins of adjacent atoms precess in tandem synchronization with one another due to the precession of the magnetization vector. The time for the precession of the spins of adjacent atoms to break synchronization is known as the transverse magnetization decay time, or “T2 relaxation time”.
One formation evaluation measurement that can be taken by NMR tools is porosity, which is defined as the fraction of the bulk volume of a formation that is not occupied by the solid framework of the formation. Thus, porosity represents the percentage of the formation (i.e. pore space) that is capable of holding fluids or gases.
A related formation evaluation measurement is fluid saturation. Saturation of a given fluid in a pore space is the ratio of the volume of that fluid to the pore space volume. For example, a water saturation of 10% means that 1/10 of the pore space is filled with water; the balance is filled with something else (oil, gas, air, etc).
Such fluid saturation measurements are therefore quite useful. Given the usefulness of fluid saturation measurements and the advantages provided by NMR tools, techniques for using NMR tools to determine fluid saturation are therefore desirable.